Field of the Invention
The present invention relates to an image processing apparatus, an image capturing apparatus, an image generation apparatus, and an image processing method, and relates particularly to a technique for correcting a distortion of a captured image that is caused by a shake of an image capturing apparatus.
Description of the Related Art
In recent years, CMOS image sensors that serve as an image sensor for use in an image capturing apparatus have become rapidly widespread. When an CMOS image sensor is used to shoot a moving image, a readout method in which accumulated electric charges are sequentially read out one line after another from the upper section to the lower section of the CMOS image sensor is widely used. This readout method is referred to as a rolling shutter method, and has a feature that readout timings are different between the upper and lower sections of the image sensor. Due to this feature, when the image capturing apparatus is shaken and the position of a subject on the image sensing surface is moved, a distortion (rolling shutter distortion) resulting from a difference in charge readout timing of the image sensor occurs in a captured image.
Various types of methods for correcting such a rolling shutter distortion by image processing have been proposed. Japanese Patent Laid-Open No. 2011-114649 discloses a technique for correcting, when a rolling shutter distortion correction amount exceeds a correction limit, only a part of the occurring rolling shutter distortion.
However, when performing control for correcting only a part of a rolling shutter distortion as described in Japanese Patent Laid-Open No. 2011-114649, the following problems will occur. That is, a rolling shutter distortion exceeds a correction limit mainly when the image capturing apparatus is in a panning state. In panning a moving image, even when only a part of a rolling shutter distortion is corrected, the residual uncorrected rolling shutter distortion is hardly noticeable under the movement of the video picture in the panning.
Meanwhile, currently, most image capturing apparatuses capable of shooting a moving image have a function to generate a still image from each of frame images constituting the moving image. When this function is used to generate a still image, the residual uncorrected rolling shutter distortion will significantly reduce the quality of the still image.